When the diet provides more than enough protein, weight maintenance is primarily determined by the phenomena involved in the interplay between intake and oxidation of carbohydrates and fats. The conditions under which carbohydrate and fat balances must be achieved differ markedly, because the body's storage capacities for glycogen and for fat are two orders of magnitude apart. The aim of this project is to examine the implications which this difference is bound to have, not only on the regulation of metabolism and its feed-back on food intake, but also in influencing body composition. By combining measurements of nutrient intake and of the 24-h respiratory exchange of mice maintained in large, hermetically sealed drums (vented once a day), the animals' carbohydrate and fat balances will be determined daily over prolonged periods. The data will permit to examine to which extent variations in food intake may reflect "corrective responses" tending to compensate for gains of losses incurred by the animals in their glycogen, fat and/or energy reserves during the preceeding day(s), and to recognize which of these appear to be the most significant for long-term weight maintenance when food is freely available. When diets containing relatively high proportions of fat are consumed, an expansion of the adipose tissue mass appears to be often necessary before the contribution of fat to the metabolic fuel mix oxidized becomes commensurate with the diets' fat content. This may be related to the fact that the unequal impact which dietary carbohydrate and fat have on metabolism also affects the regulation of body weight. This phenomenon will be examined by measuring the body and liver contents of protein fat and glycogen in mice maintained ad libitum on diets formulated to contain gradual increments in the fat-to-carbohydrate ratio. The main goal of the proposed investigations is to characterize the impact of the diet's fat content on steady-state body composition, and to delineate the relative importance of the carbohydrate, fat, and energy balances in the control of food intake. In time, this may allow to develop metabolic arguments (in addition to the current recommendations based on consideration of caloric density), to explain why a reduction in the diet's fat content may facilitate weight maintenance at a lower level of adiposity.